Avian influenza (Influenza A) viruses infect a variety of animals, including humans, pigs, horses, sea mammals, and birds. Recent phylogenetic studies of Influenza A viruses have revealed species-specific lineages of viral genes and have demonstrated that the prevalence of interspecies transmission depends on the animal species They have also revealed that aquatic birds are the source of all influenza viruses in other species.
The emergence of a “new” Influenza A virus in humans is possible. Serological and virological evidence suggests that since 1889 there have been six instances of the introduction of an influenza virus with an HA subtype that had been absent from human population for some time. Three human subtypes of HA have appeared cyclically—subtype H2 in 1889, H3 in 1900, H1 in 1918, H2 again in 1957, H3 again in 1968, and H1 again in 1977. The first human infection with avian influenza A subtype H5N1 was reported in 1997, which resulted in the death of a 3-year-old boy. This first report leads to the need for the routine screening for H5 virus in animals, particularly chicken, in stopping the spread of the viruses.
Many methods for viral identification are currently being used, including cell culture, haemagglutination-inhibition, fluorescent antibody and enzyme immunoassay, and reverse transcriptase polymerase chain reaction (RT-PCR). However, these methods all share the same problems—they have relatively low sensitivity and low specificity. Furthermore, the detection time may be too long for routine detection purposes, and such methods are relatively difficult to be utilized.
The current methodologies applied for detecting influenza A subtype H5 virus includes immunodiagnostic assay and virus culture. Examples of immunodiagnostic essay include haemagglutinin inhibition (HI) assay and immuno assay. However, immunodiagnostic assay may have the disadvantage of low sensitivity. Furthermore, as the target of immunodiagnostic assay is usually a specific protein, the underlying genetic nature of a target may not be obtained directly. In addition, the initial derivation of antibodies is ultimately dependent upon the antigenicity of the protein analysis in the immune host animal and therefore, cross-reactivity may occur.
Although virus culture is an accurate and low cost detection method, it is relatively labour intensive and requires a lot of space for incubation. The culturing process may be slow and cannot meet the demand of daily inspection. In addition, virus culture can not provide the detection results directly and has to reply upon further confirmation by other detection methods, which may be very expensive.